Checking/Adjusting Automatic Transmissions

    1. Drive the car until the transmission is sufficiently warmed up (20+ miles to be sure).
    2. Check the fluid condition and level after leaving the car idling in park for a minimum of 2 minutes.
    3. Top off if necessary, and just as importantly (if not more so), drain excess if necessary. Make sure that the transmission fluid color is normal before proceeding.
    4. Put the car into reverse. How long does it take to engage? If less than a second or two after warm, then Stu says that this is a great surrogate indicator of wear and need for rebuild. If you need norms for comparison, find a couple of MBs with known good automatics and compare against them.
    5. Test the modulator valve pressure. If you do not have the pressure gauge to test this, and particularly if you or a previous non-professional PO have "tinkered" with setting the modulator valve pressure by "feel," then spend the money and have a professional set your modulator pressure. But first, get the updated modulator valve for most early-mid 80s and beyond MBs. Why do I suggest this? Because the updated valve is cheap (~$30), has a much better "vacuum containment device" but also particularly because the "plunger" part of the valve has been updated by making it a bit longer and I can't remember why (but, I know that "it's a good thing"). And finally, the most important reason: you are a DIYer and as a fellow DIYer, we are always a hammer looking for a nail by nature, and besides, if you cannot set your own modulator pressure (since you don't have the gauge), you can at least gain the satisfaction of knowing that, by golly, you were a participant in the process of restoring your transmission to all of its former glory.
    6. If your transmission is only flaring on the 3-4 shift, that could possibly indicate trouble. At this point I'm relying on memory, but I seem to remember that this is also a pretty good surrogate indicator of potential internal wear. Having said that, though, don't assume the worst until you take care of #5 above. You see, what I suspect is happening with many early-mid 80s MBs is that more and more DIYers are beginning to acquire these cars (actually, I don't suspect this at all; it is a fact), but what I suspect is that many of these DIYers are tinkering with their modulator valves in an attempt to make the MB transmission shift as "seamlessly" as what they are used to in American transmissions. [Note that this is not a slam against MB trannies in any way. Stu has told me on several occasions that an MB tranny internally is like a fine watch as compared to the typical American tranny.] Anyway, as a result of this tinkering (which is likely done, by the way, to attempt to smooth the notoriously firm 1-2 shift), "induced slippage" is occurring with an out-of-spec (on the low side) adjustment of the modulator valve. Over time, this artificially-low modulator valve pressure, which provides short-term gains to the tinkerer in the form of non-neck-jerking 1-2 shifts, manifests itself in the way of friction plate erosion. On the other side of the pressure coin, one can, in essence, pour into their transmission "Rebuild-In-A-Can" by adjusting the modulator pressure on the way-high side. As a transmission's friction plates wear, there are increased clearances between the components, and it takes more time to "fill" these gaps. Increasing modulator pressure is an "artificial" way to reduce these clearances, and in a hurry. Unfortunately, and to use one of my favorite Stu expressions, "one can only flog a dying horse so far . . ." So, make sure that your transmission modulator valve is updated and adjusted to spec is a necessary step before proceeding to the below steps.
    7. Assuming success on the previous steps (or, determining that the transmission is on its last legs and boosting the modulator pressure to eek out some more miles), the next step is to work through the vacuum system, which necessitates (did I say necessitate? Yes) access to the workshop manual which is replete with instructions, diagrams, and so on. This vacuum system can be diagnosed using section 14, Emission Controls. I'll provide a copy of my post yesterday about my steps below that may prove useful to accompany the workshop manual. [One quick note: the modulator valve at the IP is called the "vacuum control valve" in the workshop manual for the 617 engine. Also, another quick note: from other list members' comments, this MV is about a $100 part] The first thing that I did was isolate the IP MV (by disconnecting the linkage) and connected a vacuum gauge to it (where it routes to the MV toward the tranny). With the car running, I watched the "vacuum bleed curve" on the gauge as I activated the MV. The operation of the MV was erratic, which told me that I had to open it up to try to find the problem. Also, vacuum reduction as a function of lever position (accelerator) was minimal. This device has a plastic plate (the whole housing is plastic, BTW) that is attached by 2 Phillips screws. Removal of the plate provide access to the "innards" (old Southern term; "fix" is, too, while we're on that page). The innards consist of a valve that is activated by a lever. Between the lever and the valve is a spring that serves to increase "bleed off" of the vacuum at an increasing rate. Graphically, on the horizontal axis would be the lever's operation and on the vertical axis would be bleed off. Picture a line that proceeds rightward horizontally and then starts leaping up toward the tail. Anyway, when I removed the cover, I was immediately taken aback by seeing something that looked conspicuously like JB Weld on the tip of the lever's bolted area. Sho' nuff, that's exactly what it was -- obviously applied by a PO (or a PT = previous tech) as an attempt to "lock" the lever/spring combination in a certain position. So, first step, clean up the debris and see what we had. I did so, and it looked good -- real good. The lever/bolt combination was loose such that the MV's operation was very intermittent (which explained my sometimes flare/sometimes no flare situation). I would assume that temperature of these two parts was the cause for the MV working properly sometimes. So, I decided to "crimp" the bolted area (which actually looks like a brass tube). This enabled me to precisely adjust the "bleed-off curve" (since the combination now "held" when adjusted). Note that I would recommend doing this on the car if you are in this situation. Easy to do -- just leave the cover off, hook everything up, and adjust the curve to spec.